It’s racist!
The myth that black men have bigger penises originally grew from the racist reasoning that propped up white supremacy and justified slavery in the 19th century. Black men’s oversized genitals were cited as evidence that they were “savage” and “animalistic;” outside the bounds of “normal” (read: white) sexuality and civilization. That same savage archetype — also called the “mandingo” — was summoned into the 20th century whenever white mobs wanted an excuse to lynch black men. Just claim they’re raping “our” women with their massive penises. Problem solved.